Biomedical electrode

ABSTRACT

A biomedical electrode or sensor is provided having a cup-like soft plastic base member with a solid conductive snap member therein and with a compressible sponge-like material holding a conductive paste between the solid conductive member and the body of the wearer. The device of the present invention is suitable for use as a sensor wherein it is desired to measure an electrical potential developed by the body as well as useful wherein potentials are applied to the body as in a cardiac pacemaker. The electrode was particularly designed for use with a cardiac monitor device.

United States Patent Gofman et al.

BIOMEDICAL ELECTRODE John W. Gofman; Richard F. Gianni, both of Dublin,Calif.

Cardiodynamics, lnc., Dublin, Calif.

Filed: June 13, 1974 Appl. No.: 479,112

Related US. Application Data Continuation of Ser. No. 332,720. Febv 15.abandoned.

Inventors:

Assignee:

References Cited UNITED STATES PATENTS 9/1891 Mathews 128/417 X 4/1911Noses 128/417 7/1936 Chapman 128/417 2/1957 Krasno 128/417 6/1963 Baumet a1 128/417 X 7/1969 Zeigler. Jr. et al. 339/252 P X 8/1971 Gordy128/206 E [4 1 May 13, 1975 Primary ExuminerRichard A. Gaudet AssistantE.\'aminerLee S. Cohen Attorney, Agent or Firm-Robert G. Slick [57]ABSTRACT A biomedical electrode or sensor is provided having a cup-likesoft plastic base member with a solid conductive snap member therein andwith a compressible sponge-like material holding a conductive pastebetween the solid conductive member and the body of the wearer. Thedevice of the present invention is suitable for use as a sensor whereinit is desired to measure an electrical potential developed by the bodyas well as useful wherein potentials are applied to the body as in acardiac pacemaker. The electrode was particularly designed for use witha cardiac monitor device.

8 Claims, 3 Drawing Figures PATENTED HAY 1 38s 8'. 882.853

BIOMEDICAL ELECTRODE This is a continuation, of application Ser. No.332,720, filed Feb. 15, 1973, now abandoned.

SUMMARY OF THE INVENTION It is frequently desired to apply an electrodeto a human body either for sensing a potential developed by the body asin an EKG or for applying a potential to the body as in a cardiacpacemaker. Ordinarily when an electrode is to be applied for only arelatively short period of time to a quiescent individual, no particularproblem arisesv However, if it is desired that the electrode be appliedfor long periods of time to an active individual, many problems arise.One is that the usual electrode is of such a configuration that it couldbe knocked off by normal human activities. Another is that artifactnoise is frequently developed because the potentials within theelectrode or at the skin-electrode interface change during bodilymovements. Further, there is a question of patient comfort when theelectrode must be worn for extended periods of time.

In accordance with the present invention a body electrode is pro\ idedwhich is free from artifact noise by its novel construction wherein aconductive paste is held by a sponge-like material which is undercompression between a conductive plate and the body of the wearer. Thus,as the person moves about, there is no change in potential between theelectrode and the human body, thus preventing the generation of unwantedsignals.

Thus, in the achievement of constant potential, this invention providesunique features in two major areas:

a. Maintenance of constant potential, within the electrode assemblyitself, even with stresses of body motion, pushes, and taps.

b. Maintenance of constant potential in the eleetrode-skin interface.The present invention is addressed to the maintenance of constantpotential both in (a) and (b). These are described in detail in thefollowing.

A. MAINTENANCE OF CONSTANT POTENTIAL IN THE ELECTRODE ASSEMBLY, EVENWITH STRESSES The constancy of potential of the electrode assembly iscontrolled by the constancy of ionic composition in the immediateenvironment of the metallic surface of the electrode. It is a well knownlaw of electrochemistry that the potential of metal: metal ion is alogarithmic function of the metal ion concentration in the immediateenvirons of the electrode. The present invention makes possible theachievement of constancy of metal ion ccmcentration (and activity)through the isolation of the electrode compartment from sources ofvariation of ionic composition. This is achieved by provision of anionic medium in creme or gel directly in contact with the electrodemetal and this is isolated by a porous material, such as blotting paper,from the remainder of the electrode assembly. The filling of the porousmaterial with conductive creme or gel insures electrical contact betweenthe ionic medium of the electrode compartment with the ionic medium (gelor creme) of the foam sponge which establishes the interface with thepatients skin.

The close opposition of the porous blotting material to the electrodeion compartment and its tight fit into the base itself is one keyfeature of the present inven tion which prevents movements of ionswithin the electrode compartment, hence maintaining constant ioncomposition in the immediate environment of the electrode metal.

Furthermore, the non-movement of medium within the electrodecompartment, achieved through the close and tight opposition of theporous blotting surface makes it possible to utilize a wide variety ofelectrode metals and a wide variety of metal ion concentration.including the very low concentration of metal ions achieved throughelectrode reactions, with no metal ions added. The present inventionpermits the use of all types of metals that produce a potential incontact with a conductive medium, since the isolation technique of thepresent invention removes limitations from type of metal. A wide rangeof ion concentrations is utilizable by demonstrating the invention withsilver electrodes in the absence of added silver ions (thus having onlythe silver ions generated by electrode reaction) and with silverelectrodes in contact with the very much higher silver ion concentrationgenerated by suspension of silver chloride in the creme or gel of theelectrode compartment. This is excellent proof of the efficiency offunction of the isolated electrode compartment, in maintenance ofconstant potential, even when the ionic concentration is deliberatelyvaried over wide limits. The invention, therefore, provides that withthe principle of isolation introduced, a broad variety of metallicsurfaces can be employed, and the invention therefore covers such abroad variety of metallic surfaces as electrodes.

The rigidity of the base of the central cavity, in addition to therestriction of ion movement by the porous blotting surface, is anadditional key feature which maintains constancy of ion concentration inthe vicinity of the electrode by prevention of any mass movement of gelor creme medium in the isolated electrode compartment, when theelectrode assembly is mechanically stressed.

Because of the isolation of electrode compartment via both the porousblotting surface and the rigidity of the walls of the central cavity,such stresses as pushes on the electrode, taps on the electrode, orpatient movements do not disturb the constancy of ion composition in thevicinity of electrode metal and hence constant potential is maintainedat the electrode in our invention.

B. MAINTENANCE OF CONSTANT POTENTIAL IN THE ELECTRODE-SKIN INTERFACE Wehave addressed in (a) the problem of maintenance of constant potentialwithin the electrode assembly itself, an accomplishment successfullyperformed by the present invention. Overall performance, however,demands additionally that constancy of potential be maintained at theskin-electrode interface. It is a well-known observation that anelectrical potential exists at the skin surface in humans. The precisesource of such potentials is not known. Furthermore, the potentialvaries with depth of penetration beyond the most superficial cell layersof the skin. If a constant overall potential of a skin-electrodeassembly is to be maintained, it follows that the overall assembly mustnot be permitted to sample varying potentials as a function of depthbeyond skin surface. Such varying potentials represent a source offailure of most of the biomedical sensors (electrodes) currentlyavailable.

The present invention accomplishes the maintenance of constant interfacepotential in several important ways. First, we abrade the skin gentlyeither with a sponge soaked in pumice-isopropyl alcohol (1:10 to I120pumice in alcohol by volume is satisfactory) or with gentle abrasionwith an emery board, or with gentle abrasion of an electrode creme (suchas Redux) with abrasive in the creme. All these skin preparationmodalities remove surface cornified epethclium. and bring the electrode(sensor) to the relatively deep skin layers, in which variation ofpotentials is minimal. Satisfactory performance of electrode sensors isachieved by establishment of contact with such deeper skin layers. Thepresent invention guarantees such contact through use of a resilientsponge as the immediate contact between electrode assembly and the deeplayers of skin. We have reduced to practice successful use of thissponge-under-pressure system with diverse materials, including latexrubber foam, polyurethane foam, neoprene, cellulose sponge, and cork.The key feature of this part of our invention is to have the spongesurface protrude from the electrode assembly. When the electrodeassembly is adhered to the skin, the sponge is under compression at alltimes, guaranteeing maintenance of good contact with the deep layers ofskin, where potential variation is at a minimum. To insure that thecompression does not de-adhere the electrode assembly, we use asufficiently large surface of adhesive to over balance the deadheringforce of the compressed sponge. The area chosen for adhesion dependsupon the extent to which the sponge is compressed, which in turn,depends upon the sponge density and thickness chosen.

The sponge-under-compression serves two important functions,additionally, in the present invention. First, since the sponge issoaked with electrolyte cream (and excess squeezed out), the spongeprovides a very low impedance electrical contact path from skin to theelectrode compartment. The sponge is under compression and hence at alltimes in excellent contact with the porous blotting surface of theelectrode compartment as well as in excellent contact with skin. Thisfeature of the invention obviates an important source of artifact noise,namely that due to make-break phenomena within the electrode assembly orat the skin electrode interface, especially when the electrode assemblyis stressed by patient motion, by pushes on the assembly or by taps onthe assembly.

The present invention has further features which prevent potentialvariation at the skin-electrode interface. It is essential that nomechanism of stress should alter the skin-electrode interface, sincethis would give rise to observation of varying potentials such varyingpotentials being the essence of artifact noise." The usually availablecommercial sensors are deficient in this respect in that pressurestresses on the electrode are directly transferred to the skin-electrodeinterface. In this way potential variation is minimized or eliminated,protecting the assembly from this source of artifact noise.

There is still another feature of the invention which is important forminimizing voltage artifacts. The sponge, while soaked in gel or creme,is then squeezed out to eliminate excess creme or gelv Thus, theinvention uses a semi-dry" sponge, in that no excess creme is availablewhen the electrode is mounted on the skin. This feature is important,for if excess creme or gel is present, in the usual wetsponges, suchexcess creme can find its way between the electrode and skin includingseparation of the adhesive surface from skin. Once this situation isreached, pressure stresses on the electrode assembly transfer the stressto the creme-skin contact and force this creme to contact with deeperlayers of skin, resulting in the sensing of the potential variation withdepth. This is a source of artifact voltage. The semi-dry" sponge doesnot provide such excess creme or gel and hence prevents this describedsource of artifact voltage.

The invention has been successfully employed with a variety ofelectrolyte cremes and gels, so that variation in gel or creme does notconstitute a departure from our invention. Among these we havesuccessfully utilized:

EKG Solution Redux Creme Redux Paste Ferris Gel Cam-Creme This widevariety of cremes and gels will all work satisfactorily with theinvention.

Further, the construction of the electrode base of the present inventionis such that it is made of a soft plastic which conforms to the body ofthe wearer so that it can be worn for long periods of time Withoutdiscomfort.

Another feature of the invention is that the base member is shaped insuch a manner that lateral forces, which might otherwise detach theelectrode from the body, are deflected.

Another feature of the present invention is that it can be made of twoinjection molded parts at low cost so that it can be considered anexpendable item.

Other features of the invention will be brought out in the balance ofthe specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, partlyin section, of a body electrode embodying the present invention.

FIG. 2 is an enlarged section on line 22 of FIG. 1.

FIG. 3 is an exploded perspective view of an electrode embodying thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings byreference characters, the electrode of the present invention isgenerally designated 5 and consists of two main parts, namely, a base 7and snap 9.

The base includes a wide rim portion 11 which is relatively thin so thatit can conform to the shape of the body, particularly during movement.The base includes a central cavity 13 which is defined by the sidewall15 and the top 17, both of which are relatively heavy construction. Itwill be noted that both the sidewall 15 and top 17 slant to form aramplike configuration which aids in retaining the device in thepresence of lateral forces. At the center of the top a hole 19 isprovided for the reception of the center post of the snap, laterdescribed. In the cup-like inner portion 13 are four bosses 21 whichserve to retain the snap within the cup. The base thus far described ispreferably cast from a single piece of flexible polyvinylchloride,although it could be made out of natural or artificial rubber or anyrelatively soft plastic. A double sided adhesive 25 is applied to thelower surface of rim 11 for attachment of the base to the body of thewearer.

The snap 9 includes a disc-like member 27 having a sharp edge 29. At thecenter, a post 31 is formed having a bulbous top 33. The bulbous top 33is bifurcated as at 35 which aids in snapping the electrode to astandard clip, and which also facilitates removal of the snap piece fromthe injection molding tool (if the part is made by injection molding).Preferably. the snap is made of a relatively hard plastic which isplatable such as No. EP-3510 acrilonitrile butadiene styrene (ABS) whichis plated successively with copper. nickel and then silver. The snapcould be cast from metal but the plastic is lighter weight, lessexpensive and provides excellent conductivity when made in this manner.The sharp edge 29 of the snap facilitates snapping it over the bosses 21of the base and also acts as a seal, preventing leakage of the electrodejell solution out of the inner cup chamber of the base. The bosses 21retain the snap 9 into the base 7. This retaining could be performed byany undercut in the inner cup chamber of the base 7.

In use, a disc 37 of porous paper such as blotting paper and arelatively thick disc 39 ofa foamlike material such as latex orpolyurethane foam is provided. The paper 37 and the foam 39 are bothsaturated with an electrode jell solution and, as can best be seen inFIG. 2, the foam extends slightly beyond the bottom edge of the flatportion 11. Thus, when the electrode is applied to the body, the discs37 and 39 are under compression, maintaining a constant resistancebetween the body and the snap, even during vigorous body movement. Thepaper disc 37 and foam disc 39 are attached to each other with anadhesive. Since the paper fits tightly into the cup chamber of the base7, the foam disc 39 is also securely retained into the assembly. Sincethe sides 15 and top 17 form ramp-like members, the electrode is highlyresistant to lateral forces which might otherwise detach the electrodefrom the body.

It will be apparent to those skilled in the art that many departures canbe made from the exact structure shown without departing from the spiritof this invention.

We claim:

1. A body electrode comprising in combination:

a. a base member made of a single piece of a flexible,

soft plastic having a thin flexible rim portion and a rigid center flatbottomed cup-like member defined by heavy sidewalls and a heavy top,

b. a snap comprising a relatively thin flat disc of conductive materiallying on the flat bottom of the cup-like member, and having a terminalextending through the face of the cup-like member.

c. a thin sheet of porous non-conductive material of substantially thesame size as the flat bottom of the cup-like member. and having onesurface in contact with said disc.

(1. a thick disc of porous foam-like material in contact with saidporous nonconductive material and filling the remainder of the cavity ofthe cup-like member and extending a substantial distance beyond the openend of the cup-like member, said foam-like material being saturated withan electrically con ductive material, and

e. means for attaching said cup to a human body by the outer thin widerim whereby said foam-like material is brought into compression withinthe cuplike member by pressure from the body.

2. The structure of claim 1 wherein the outer surface of said cup-likemember is formed at an angle to provide a ramp to resist lateral forces.

3. The structure of claim 1 wherein said rim is provided with a doublesided adhesive on its outer surface surrounding the opening of the cupfor attachment of the electrode to the body.

4. The structure of claim 1 wherein the inner surface of said cup isprovided with a plurality of bosses, said snap being retained betweenthe bosses and the flat bottom of the cup-like member.

5. The structure of claim I wherein the snap terminal is a center postextending through the base, said center post terminating in a bifurcatedbulbous member.

6. The structure of claim 1 wherein said snap has a sharp edge at theperimeter of said disc for retaining itself within the base and actingas a seal.

7. The structure of claim 1 wherein the thin sheet of porousnonconductive material is a sheet of paper.

8. The structure of claim 7 wherein the porous paper consists of a sheetof blotting paper and the foam-like material consists of a disc ofpolyurethane foam.

1. A BODY ELECTRODE COMPRISING IN COMBINATION: A. A BASE MEMBER MADE OFA SINGLE PIECE OF A FLEXIBLE, SOFT PLASTIC HAVING A THIN FLEXIBLE RIMPORTION OF RIGID CENTER FLAT BOTTOMED CUP-LIKE MEMBER DEFINED BY HEAVYSIDEWALLS AND A HEAVY TOP, B. A SNAP COMPRISING A RELATIVELY THIN FLATDISC OF CONDUTIVE MATERIAL LYING ON THE FLAT BOTTOM OF THE DUP-LIKEMMEMBER, AND HAVING A TERMINAL EXTENDING THROUGH THE FACE OF THECUP-LIKE MMEMBER, C. A THIN SHEET POROUS NON-CONDUCTIVE MATERIAL OFSUBSTANTIALLY THE SAME SIZE AS THE FLAT BOTTOM OF THE CUP-LIKE MEMBER,AND HAVING ONE SURFACE IN CONTACT WITH SAID DISC, D. A THICK DISC OFPOROUS FOAM-LIKE MATERIAL IN CONTACT WITH SAID POROUS NONCONDUCTIVEMATERIAL AND FILLING THE REMAINDER OF THE CAVITY OF THE CUP-LIKE MEMBERAND EXTENDING A SUBSTANTIAL DISTANCE BEYOND THE OPEN END OF THE CUP-LIKEMEMBER, SAID FOAM-LIKE MATERIAL BEING SATURATED WITH AN ELECTRICALLYCONDUCTIVE MATERIAL, AND E. MEANS FOR ATTACHING SAID CUP TO A HYMAN BODYBY THE OUTER THIN WIDE RIM WHEREBY SAID FOAMM-LIKE MATERIAL IS BROUGHINTO COMPRESSION WITHIN THE CUP-LIKE MEMBER BY PRESSURE FROM THE BODY.2. The structure of claim 1 wherein the outer surface of said cup-likemember is formed at an angle to provide a ramp to resist lateral forces.3. The structure of claim 1 wherein said rim is provided with a doublesided adhesive on its outer surface surrounding the opening of the cupfor attachment of the electrode to the body.
 4. The structure of claim 1wherein the inner surface of said cup is provided with a plurality ofbosses, said snap being retained between the bosses and the flat bottomof the cup-like member.
 5. The structure of claim 1 wherein the snapterminal is a center post extending through the base, said center postterminating in a bifurcated bulbous member.
 6. The structure of claim 1wherein said snap has a sharp edge at the perimeter of said disc forretaining itself within the base and acting as a seal.
 7. The structureof claim 1 wherein the thin sheet of porous nonconductive material is asheet of paper.
 8. The structure of claim 7 wherein the porous paperconsists of a sheet of blotting paper and the foam-like materialconsists of a disc of polyurethane foam.